NDTnet - May 1997, Vol.2 No.05
Ultrasonic imaging of internal defects in composites
DLR Braunschweig, Germany
Presented on the UTonline Application Workshop in May '97
- High resolution ultrasonic scanning system
- Inspections of thin CFRP laminates
- Inspections of sandwich components
High performance materials such as carbonfiber-reinforced plastics (CFRP) and sandwich components are attractive materials for aircraft and aerospace components. Their application to primary aircraft structures requires the knowledge of damage incurred after fabrication or in service [1,2]. CFRP-laminates are nonhomogeneous and anisotropic materials; a 2 mm laminate, for example, may consist of 16 layers of fibres with a thickness of 0.125 mm each. The sequence of stacking is determined by design requirements. Ultrasonic attenuation in composites is relatively high, and the scatter by the fibres reduces the signal to noise ratio. The time of flight of a 2 mm thick laminate is only 1.3 µs, therefore a high axial resolution of the flaw detector is required . The thickness of CFRP-components may vary from 2 to 40 mm, the one-shot dynamic range of a through-transmission measurement easily reaches more than 60 dB .
2. High resolution ultrasonic scanning system
For composites and other new materials an ultrasonic imaging system has been developed . The computer controlled ultrasonic flaw detector HFUS 2000 provides high resolution inspections in the frequency range from 0.1 to 120 MHz and is of modular design. The amplitude and the time of flight can be evaluated within three gates. The single-shot peak-detector 2051 provides two amplitude measurements in two different gates in 32 1-dB steps in the high frequency mode (up to 120 MHz), or one amplitude measurement in 64 1-dB steps (up to 10 MHz).
The transmitter pulse is generated by avalanche transistors with a rise time of 125,000 V/µs. This technique allows the possibility of generating ultrasonic broadband pulses with frequencies up to 120 MHz . Together with a preamplifier, the pulser is located in a separate case in order to shorten the cable to the transducer. This system is used at DLR in Braunschweig with a three axes manipulator with tank (scanning area of 400 mm * 400 mm including a Z-axis of 100 mm).
The US-SCAN 2.0 software not only provides the manipulation and the settings of the ultrasonic flaw detector, but also the storage of data and imaging of test results in A-, B-, C- and D- scans. The scanning system measures up to 20,000 amplitudes and 10,000 time-of-flight values per second. The results can be plotted in 16 different grey levels (corresponding to the dB-steps of the peak detector) by a laser printer, or in 16 pseudo colours. A real-time contrast enhancement and other forms of imaging processing give clear presentations of results.
The HFUS 2000 ultrasonic equipment is connected via LAN to a file server. Two other computers are also combined via LAN to this NDI-file server. One of them is designed for image enhancement. The ultrasonic results can be displayed and evaluated on these computers while the ultrasonic system is controlling the ultrasonic inspections.
3. Inspections of thin CFRP laminates
For the inspections of thin CFRP laminates in immersions technique a 50 MHz PVDF-foil-transducer was chosen. The advantages of this type of transducer are the large bandwidth and a focussing through the deformation of the foil. Therefore no separate acoustic lens with delay line is required. The selected transducer with an element diameter of 3 mm and a focal length of 50 mm in water gives a sound beam diameter of 0.25 mm. This relatively long focal length allows a 20 mm long test range in water, which means a constant sound beam diameter for 2 mm thick CFRP laminates.
Figure 1: Pulse response of a 50 MHz PVDF-foil transducer, scale: 0.1 V/div. and 50 ns/div.
right Figure 2: Frequency spectra of a glassplate reflector,
1: Broadband, 2: with B-scan filter, 3: with C-scan filter, scale: 1MHz/div.; 4 dB/div.
The large bandwidth provides a short echo pulse shown in Fig. 1. The water delay and the CFRP-specimen especially damp high frequencies. Therefore the maximum test frequency of the 50 MHz transducer is shifted to 10 MHz (!) (see Fig. 2, curve 1). The signal-to-noise ratio is high enough, so that the loss of high frequencies can be compensated by a receiver filter unit .
In order to improve the identification of matrix cracks in CFRP-laminates, a "C-scan" filter has been developed which selects a frequency range with 27 MHz centre frequency (see curve 3 in Fig. 2). This frequency represents ultrasonic sound waves with a wave length of about 0.13 mm and increases the amplitude changes caused by matrix cracks up to 12 dB.
Figure 3: C-scan recorded without C-filter. right Figure 4: C-scan recorded with C-filter
Typical results without and with the "C-scan" filter are shown in Figs. 3 and 4. Fig. 3 presents the C-scan in double through-transmission technique without filter for a 2 mm thick CFRP-specimen (DLR, lay up : [ 0°, +45°,- 45°, 90°, 0°,+ 45°,- 45°, 90°] s ), which was loaded in fatigue (N= 67710, Density= 200 N/mm2, R = -1). The damage of this specimen consists of matrix cracks and edge delaminations. Fig. 4 shows a C-scan with receiver filter from the same specimen. The cracks are much better visible.
The B-scan filter (curve 2 in Fig. 2) increases the axial resolution and provides high resolution B-scans. An example is shown in Fig. 5 which presents a B- scan of a 2 mm thick laminate with impact damage. The impact produced delaminations in different depths which are displayed by black lines in a range of 18 to 18.7 µs. The back-wall echo is situated at 19 µs. In defect-free regions between the interface- and the back- wall echo 15 layer echoes are displayed. This high resolution is useful for damage analysis as well as for the control of repairs. || Fig. 5|
High resolution B- scan of an impacted 2mm thick CFRP-laminate
4. Inspections of sandwich components
The ultrasonic imaging technique for honeycomb-sandwich components must be capable of detecting all kinds of defects in skin and core not only for laboratory use, but also for field-inspections of 'real components' as well . Typical damages to be detected are: cracks and delaminations in the skin, debondings between skin and core and defects in the core (crushing), which are seldom visible from the outside.
The attenuation of sandwich components increases very much at high frequencies. Therefore only frequencies below 1 MHz can pass through the whole thickness of these components. System components such as transmitter (pulser) and receiver have to be optimised in order to obtain a high signal-to-noise ratio which are necessary for high quality results. Such optimisations of the pulse parameters have been already carried out for sandwich components with Nomex-cores and CFRP skins (table 1) .
| Table 1: Description with pulse parameters of the sandwich specimen D
|| CFRP-prepreg / Nomex -core
| total thickn./ thickness of core ||16 mm/15 mm
| dimensions || 310*100 mm
| time of flight through tot. Thickness. ||13.7 µs
| core velocity || 2300 m/s
| skin velocity||3100 m/s|
| pulse width of pulser
RF-A-scans with interface-and back wall echo|
| Spectrum of back wall echo ||
| Frequency range (-6 dB)||0.42-0.71 MHz |
Bandwidth (-6 dB)||0.71 MHz |
| Maximum ||0.8 MHz
The test frequencies for a single skin have to be higher than 15 to 25 MHz in order to separate the interface and the back- wall echo (0.5 to 1.0 mm thick). For the inspection of the whole sandwich, a special pulser/receiver module for the HFUS 2000 ultrasonic flaw detector was developed. This module consists of a rectangle pulser with pulse-width settings from 0.1 to 2.0 µs and a broadband receiver with a 0.3 MHz high pass and a 2.0 MHz low pass filter. The rectangle pulser delivers a high output of energy in a frequency range which can be optimised by the pulse width setting. The filters of the receiver suppress the low frequencies which do not detect core deficiencies, and the low pass filter reduces the scattering and electronic noise. This pulser/receiver unit allows the application of the echo-technique with broadband transducers .
Specimen "D1" with several defects has been used for parameter optimisation. Defect "N", a "natural" defect caused by static loading is situated around the bore hole (25 mm dia. ). Other defects are artificially inserted : "B" and "C" are horizontal cuts in the core, "B" in front of the back skin and "C" in the centre of the core. Defect "D" is a diagonal cut in the core. Table 1 gives more details about the specimen and the pulse parameters.
| Fig. 6: C-scan of a honeycomb specimen with internal defects || Fig. 7: Horizontal B-scan with defects "C", "N" and "D" |
Figure 7 is a B-scan from specimen "D1", recorded along a vertical line extended through defects "C", "N" and "D". The defects are indicated by interruptions of the back-wall echo (16 µs on the time scale) and echo amplitudes between interface and back-wall echo: defect "C" between x=10 and x= 30 mm with echo amplitudes between 8 and 12µs, defect "N" at x = 100 mm and between 4 and 8 µs and defect "D" between x=120 mm and x=160 mm indicating a diagonal direction indication.
Figure 6 presents a C-scan in echo technique of specimen "D1" with back-wall echo evaluation. The all defects "B", "C", "D" and "N" are clearly displayed by amplitude decreases of -8 to -15 dB. Optimised pulse parameters were used.
Ultrasonic imaging of internal defects with a high degree of validity in composite components requires an optimisation of the pulse parameters. Broadband transducers provide short pulses for high axial resolution and transmit a wide frequency range. With receiver filters it is possible to select frequencies which provide best defect indications. For C-and B-scans different frequency spectra from one transducer can be selected: one for best lateral and another for high axial resolution. This paper describes two applications of this technique: (1) the inspection of thin CFRP-Laminates and (2) those of honeycomb components with Nomex cores and CFRP-skins. For all inspections the HFUS 2000 flaw detector is used. This modular system was optimised for these different applications.
For in-field inspections the MUSE (Mobile ultrasonic equipment) with its motor-driven manipulator provides ultrasonic imaging of components with optimised pulse parameters . The coupling is carried out with a water circulation system.
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